In the thermal cracking of hydrocarbons, relatively light starting materials such as ethane or propane, or hydrocarbon mixtures with a boiling point below 200.degree. C., for example naphta, are generally preferred since these starting materials produce fewer byproducts. However, these starting materials are not always available in sufficient quantity and hence it is frequently necessary or desirable to subject heavier starting materials to pyrolytic or thermal cracking. In general, therefore, efforts have been made to develop processes for cracking high boiling starting materials and thereby rendering the same more useful.
The higher boiling starting materials, however, have been found to be problematic when subjected to thermal cracking since the cracking operation yields relatively large quantities of liquid cracking products in amounts which increase with increasing boiling point range of the starting materials. These liquid cracking products are generally separated into a fraction boiling below 200.degree. C. and a fraction boiling above 200.degree. C.
The lower boiling fraction constitutes a relatively high octane fuel and contains valuable components such as benzene, toluene and xylenes. By contrast, the higher boiling fraction, i.e. the fraction with a boiling range above 200.degree. C., is a product difficult to treat or render useful and contains highly condensed aromatics, polymeric compounds sulfur compounds and heavy metal compounds. The proportion of this fraction, in the case of naphta cracking, can be between 1 and 5% by weight of the total product and with heavier starting material for the cracking operation, it increases so that in the case of gas oil, for example, it can amount to 30 weight percent with still higher values when the material subjected to cracking is vacuum gas oil or crude oil or a crude oil residual such as residual oil.
Furthermore, the sulfur originally present in the material subject to cracking is enriched in the heavy product fraction to such an extent that the combustion of the product fraction with dilution or treatment to remove sulfur, poses a serious environmental hazard.
Consequently, efforts have been made to cut the high-sulfur cracking-product fraction with low-sulfur fuels to avoid combustion gases which are toxic because of the sulfur content. The mixture of the heavy-cracking-product fraction with low-sulfur fuels, however, also poses problems since this fraction is only limitedly miscible with crude oil distillates and thus can only be cut in part by them. Another disadvantage of the heavy cracking product fraction is that it is difficult to store, handle and transport.
An economical process for working up or treating this heavy cracking-product fraction to render it more useful and to avoid the aforementioned problem has not been proposed heretofore.
For example, German Patent Document No. P 28 06 854.4 suggests that the cracking-product fraction boiling above 200.degree. C. can be subjected to a treatment in which polymeric compounds are removed with the polymer-free fraction by hydrogenating it and the hydrogenated product subjected to thermal cracking. This process, however, requires uneconomically large quantities of hydrogen if the hydrogenated product is to have a quality which enables it to be useful economically as a feed for the cracking process.